1. Field of the Invention
The present invention relates to OFDM (Orthogonal Frequency Division Multiplexing) receiving apparatuses for receiving OFDM-modulated signals for use in terrestrial digital television broadcasts. In particular, the present invention relates to an OFDM receiving apparatus, suitable for on-board use in mobile units, in which delayed waves contained in an OFDM-modulated signal are removed to reduce the bit error rate after demodulation.
2. Description of the Related Art
The structure and operation of an OFDM receiving apparatus of the related art are described below with reference to FIGS. 7 and 8. FIG. 7 illustrates the overall OFDM receiving apparatus, and FIG. 8 specifically illustrates a delay equalizer in the OFDM receiving apparatus shown in FIG. 7.
An OFDM-modulated signal received by an antenna 11 is amplified and frequency-converted in a receiver 12. The resulting signal is further converted into a pair of digital baseband signals, which stand in orthogonal relation to each other, by an analog-to digital (A/D) converter (not shown). It is assumed herein that the received OFDM signal contains a direct wave directly delivered from a transmission antenna, and a delayed wave delivered after being reflected by an obstruction such as a building, and the converted digital baseband signals also include a direct wave component and a delayed wave component. In the following description, however, the overall digital baseband signals are simply referred to as an “OFDM-modulated signal”, unless specifically noted.
The OFDM-modulated signal output from the receiver 12 is input to an autocorrelation detector 13, a path correlation detector 14, and a delay equalizer 15.
One transmitted symbol of the OFDM-modulated signal is formed of a guard interval period in which it is tolerant of interference of the delayed waves, and an effective symbol period in which an information signal of interest to be transmitted is inserted.
The autocorrelation detector 13 detects the periodicity of the OFDM-modulated signal. For this purpose, the OFDM-modulated signal is input to both an effective symbol time delay unit 13a and a complex conjugate signal generating unit 13b. The effective symbol time delay unit 13a delays the input OFDM-modulated signal by the effective symbol period. The complex conjugate signal generating unit 13b generates a signal (complex conjugate signal) that is the conjugate of the input OFDM-modulated signal (one of the pair of orthogonal digital baseband signals is used as the real part and the other as the imaginary part). The OFDM-modulated signal delayed by the effective symbol time delay unit 13a and the complex conjugate signal generated by the complex conjugate signal generating unit 13b are multiplied by a multiplying unit 13c. 
The multiplication results of the multiplying unit 13c are accumulated for a predetermined time by an accumulating unit 13d, and the value obtained as a result of accumulation is an autocorrelation signal which iterates every transmitted symbol. There are two types of autocorrelation signals, i.e., a real-part autocorrelation signal and an imaginary-part autocorrelation signal, as described above. The imaginary-part autocorrelation signal is significantly smaller and varies for one transmitted symbol period less than the real-part autocorrelation signal. The real-part autocorrelation signal exhibits some peaks for one transmitted symbol period of the OFDM-modulated signal, which are used to detect the periodicity of the OFDM-modulated signal. An autocorrelation peak (global maximum) that appears first corresponds to the amplitude of the direct wave, and an autocorrelation peak (local maximum) that appears later than the first peak corresponds to the amplitude of the delayed wave. A plurality of delayed waves produce a plurality of local maxima. Since the delayed wave arrives at the antenna 11 later than the direct wave, the delay time of the delayed wave with respect to the direct wave can be found based on the difference between the time when the global maximum appears and the time when the local maximum appears.
The path correlation detector 14 detects the phase difference between the direct wave and the delayed wave which is produced due to the difference in transmission route (path) therebetween. For this purpose, the OFDM-modulated signal is input to both a delay time delay unit 14a and a complex conjugate signal generating unit 14b. The delay time delay unit 14a delays the input OFDM-modulated signal by the delay time of the delayed wave with respect to the direct wave (the above-described delay time). The complex conjugate signal generating unit 14b generates a signal (complex conjugate signal) that is the conjugate of the input OFDM-modulated signal (one of the pair of orthogonal digital baseband signals is used as the real part and the other as the imaginary part). The complex conjugate signal generating unit 14b has the same structure as that of the complex conjugate signal generating unit 13b of the autocorrelation detector 13. The OFDM-modulated signal delayed by the delay time delay unit 14a and the complex conjugate signal generated by the complex conjugate signal generating unit 14b are multiplied by a multiplying unit 14c. 
The multiplication results of the multiplying unit 14c are accumulated for a predetermined time by an accumulating unit 14d, and the value obtained as a result of accumulation is a path correlation signal. There are also two types of path correlation signals, i.e., a real-part path correlation signal and an imaginary-part path correlation signal, as described above. Both the real-part and imaginary-part path correlation signals exhibit a substantially constant path correlation value for one transmitted symbol period such that the arc tangent of mean path correlation value Ip for the real part and mean correlation value Qp for the imaginary part, i.e., arc tan Qp/Ip, represents the phase difference between the direct wave and the delayed wave.
When the delayed wave contained in the OFDM-modulated signal is delayed longer than the guard interval period (which has been inserted during modulation) in one transmitted symbol, the delay equalizer 15 removes the delayed wave and outputs only the direct wave. The structure of the delay equalizer 15 is shown in FIG. 8. The delay equalizer 15 includes an adding unit 15a having a first input terminal (+) to which the OFDM-modulated signal containing the delayed wave is input, and a feedback unit 15b connected between the output terminal of the adding unit 15a and a second input terminal (−) of the adding unit 15a. The feedback unit 15b has a delay unit 15b1 and a complex amplitude correcting unit 15b2.
The delay equalizer 15 further includes a global/local maxima autocorrelation search unit 15c to which the autocorrelation signal is input, and two calculating units, namely, a delay time calculating unit 15d and a complex amplitude coefficient calculating unit 15e, which are connected to the global/local maxima autocorrelation search unit 15c. 
The global/local maxima autocorrelation search unit 15c searches for the times T1 and T2 at which the global and local maxima of the input autocorrelation signal appear (the global and local maxima may be the global and local maxima Ia and Ib with respect to the real-part autocorrelation signal, respectively), and sends a signal containing this information to the delay time calculating unit 15d. The delay time calculating unit 15d calculates the delay time (T2−T1) of the delayed wave based on the times T1 and T2, and sends a signal containing this information to the delay unit 15bl. The signal of the determined delay time (T2−T1) is also sent to the delay time delay unit 14a of the path correlation detector 14 to define the delay time thereof.
The delay unit 15bl delays the OFDM-modulated signal output from the adding unit 15a, which substantially includes the direct wave alone with the delayed wave removed therefrom, by the delay time (T2−T1) of the delayed wave. The delay unit 15b1 then inputs the result to the complex amplitude correcting unit 15b2. Thus, the direct wave negatively fed back to the adding unit 15a coincides in time with the delayed wave of the OFDM-modulated signal initially input to the adding unit 15a. 
The global/local maxima autocorrelation search unit 15c further searches for the global maximum Ia of the real-part autocorrelation signal and the global maximum Qa of the imaginary-part autocorrelation signal (which may be substituted for the mean values thereof), and sends the results to the complex amplitude coefficient calculating unit 15e. 
Also input to the complex amplitude coefficient calculating unit 15e is the path correlation signal (real-part path correlation value Ip and imaginary-part path correlation value Qp) from the path correlation detector 14. The complex amplitude coefficient calculating unit 15e calculates a complex amplitude coefficient based on the above-noted four correlation values (Ia, Qa, Ip, and Qp). The complex amplitude correcting unit 15b2 uses the complex amplitude coefficient to correct the complex amplitude of the direct wave output from the delay unit 15b1 so as to coincide with the complex amplitude of the delayed wave of the OFDM-modulated signal input to the adding unit 15a. 
As mentioned above, in the OFDM receiving apparatus of the related art, the delay time between the direct wave and delayed wave of the received OFDM-modulated signal is determined, and the complex amplitude of the fed back direct wave coincides with the complex amplitude of the delayed wave, thereby removing the delayed wave. However, noise superposed on the autocorrelation signal would cause a change of the time at which a low-level delayed wave peak (local maximum) appears, thus producing an error in the determined delay time to fail to determine the correct delay time. For example, as shown in FIG. 9, the peak, which must appear at time t for determining the delay time of the delayed wave, appears at time t1 due to superposition of noise, thus causing an error of difference δ between the obtained delay time t0 and the true delay time.